The delayed coking process has evolved with many improvements since the mid-1930s. Essentially, delayed coking is a semi-continuous process in which the heavy feedstock is heated to a high temperature (between 900° F. and 1000° F.) and transferred to large coking drums. Sufficient residence time is provided in the coking drums to allow the thermal cracking and coking reactions to proceed to completion. The heavy residua feed is thermally cracked in the drum to produce lighter hydrocarbons and solid, petroleum coke.
The product mixture resulting from the coking process may be affected by the cracking temperature, including heater outlet conditions and coke drum conditions. One of the initial patents for this technology (U.S. Pat. No. 1,831,719) discloses “The hot vapor mixture from the vapor phase cracking operation is, with advantage, introduced into the coking receptacle before its temperature falls below 950° F., or better 1050° F., and usually it is, with advantage, introduced into the coking receptacle at the maximum possible temperature.” The “maximum possible temperature” in the coke drum favors the cracking of the heavy residua, but is limited by the initiation of coking in the heater and downstream feed lines, as well as excessive cracking of hydrocarbon vapors to gases (butane and lighter). When other operational variables are held constant, the “maximum possible temperature” normally minimizes the volatile material remaining in the petroleum coke by-product. In delayed coking, the lower limit of volatile material in the petroleum coke is usually determined by the coke hardness. That is, petroleum coke with <8 wt. % volatile materials is normally so hard that the drilling time in the decoking cycle is extended beyond reason. Various petroleum coke uses have specifications that require the volatile content of the petroleum coke by-product be <12%. Consequently, the volatile material in the petroleum coke by-product typically has a target range of 8-12 wt. %.